Generally, in these devices, the piezoelectric effect involved is reversible and a deflection can be measured by a piezoelectric gauge. Such a gauge can be produced on a piezoelectric actuator by photolithography. One portion of the top electrode serves as a sensor and the other as an actuator. This is what has been produced by T. Kobayashi et al. “A fatigue test method for Pb(Zr,Ti)O3 thin films by using MEMS-based self-sensitive piezoelectric microcantilevers”, Kobayashi T., Maeda R., Itoh T., National Institute of Advanced Industrial Science and Technology, Journal of Micromechanics and Microengineering, 18 (2008), 115007, 6 pages, and which is illustrated in FIG. 1 which shows a piezoelectric gauge 11 associated with a piezoelectric actuator 10.
The density of charges generated by the piezoelectric material is proportional to the strain applied to the material. The greatest strains are located at the fixed end. Furthermore, the total quantity of charges recovered is also proportional to the surface area of the electrode. To recover the maximum of charges at a fixed level of strains, it is therefore essential to be situated at the fixed end and have the greatest possible electrode surface area. However, if the surface area dedicated to measuring the deflection is increased, the surface area used for actuation is reduced accordingly. Furthermore, if the maximum deflection is required, piezoelectric actuation at the fixed end must be favoured.
In this configuration, the deflection measurement and the actuation compete strongly. In order to produce devices in which the actuation and the deflection measurement are both fully integrated, it is necessary to use another type of gauge for the deflection measurement.
To this end, it is also known to use integrated piezoresistive gauges, the deflection measurement by such strain gauges being notably highly advantageous in micro- and nano-electromechanical systems (MEMS and NEMS), because the latter can be fully integrated and are therefore easy to produce. These gauges are used in all kinds of sensors (chemical, biological, inertial, etc.) and can be of reduced dimensions since the signal is no longer proportional to the surface area of the gauge, but to its length.
The expression “piezoresistive gauge” should be understood to mean a gauge whose electrical resistance is modified by the action of a mechanical strain due to a deformation.
Most of the applications use piezoresistive gauges based on p-doped silicon. The gauges are directly produced on the silicon substrate. This material makes it possible to have very high gauge factors (typically greater than 20). However, they are highly sensitive to temperature variations, which increases thermal noise. Their resistance is also very high, which limits the current that can be passed through these gauges. Furthermore, producing these gauges involves complicated and expensive fabrication techniques (ionic and/or epitaxial implantation). It is this type of gauge that was used by J. Lu et al., “High-Q and CMOS compatible single crystal silicon cantilever with separated on-chip piezoelectric actuator for ultra-sensitive mass detection”, Lu J., Ikehara T., Zhang Y., Mihara T., Itoh T., Maeda R., National Institute of Advanced Industrial Science and Technology, MEMS 2008, Jan. 13-17 2008, Tucson, Ariz., USA, and presented in FIG. 2. More specifically, according to this type of device that uses a piezoresistive gauge, an actuator comprises a piezoelectric material 22 contained between a bottom electrode 23 and a top electrode 24, the silicon substrate being in a region 25 that is locally doped, for example, by boron ions, all of the contact points 26 typically being able to be provided by an Au/Cr bilayer, the actuator being protected by a layer 27 of SiO2 type.
Metal strain gauges have also already been proposed for certain applications. The gauge factor is then weaker than for the silicon gauges (typically of the order of 2), but the resistance of the gauge is lower, which makes it possible to increase the current density. The same signal level is thus obtained as with silicon gauges. The measurement noise obtained is also limited, which makes it possible to obtain a sensitivity equivalent to that of piezoresistive gauges. Such is notably the subject of US patent 2009/003804 by H. Tang et al.: “Metallic thin film piezoresistive transduction in micromechanical and nanomechanical devices and its application in self-sensing SPM probes”, Tang H., Li M., Roukes M. L., California Institute of Technology, US 2009/0038404 A1, an exemplary application of which is the atomic force microscope presented in FIG. 3.
FIG. 3 is the diagram of an atomic force microscope tip using a piezoelectric actuation 5 and a measurement of deflection of the tip 1 by a metal strain gauge 9.
The production of these gauges however entails costly microelectronic production steps that are not without influence on the device.
It is therefore best to find a way of measuring the deflection of a piezoelectric actuator to produce an all-integrated device without in any way making its production more difficult.
In practice, in summary, the solutions of the prior art propose piezoelectric actuator stacks of the following type: elastic layer/bottom electrode/piezoelectric material/top electrode, as illustrated in FIG. 1, and to integrate the deflection measurement, either to separate the piezoelectric stack into a region used for actuation and a region used for measurement, or to use a piezoresistive gauge that may be made of silicon.
In the case of a piezoelectric gauge like that illustrated in FIG. 1, it is best to make a trade-off between the surface area dedicated to measurement and the surface area dedicated to actuation.
Moreover, the piezoresistive gauges and the metal strain gauges are produced by specific and costly technological steps, and increase the stack complexity. In practice, the known piezoelectric actuators that include a metal strain gauge are produced by a stack of layers independently producing the actuator and the strain gauge. The latter is, for example, produced by a metal layer arranged under the elastic layer of the stack producing the piezoelectric actuator.